System for transmitting electrical signals



July 16, 1940. J L, T Z 2,207,794

SYSTEM FOR TRANSMITTING ELECTRICAL SIGNALS Original Filed Oct. 12. 1936Patented Jul is, 1940 PATENT OFFICE SYSTEM FOR TRANSMITTING ELECTRICALSIGNALS James L. Getal, Maryvllle, Tenn.

Application October 12, 1936. Serial No. 105,229 Renewed December 27,1938 Claim.

This invention relates to the transmission of electrical signals andparticularly to the control of the volume of the signals beingtransmitted.

The purposes of this invention are as follows:

1. To automatically contract the range of amplitude of the signal waves,so that this amplitude may be made as nearly constant as desired for allvolumes of the sound, which is being transmitted.

2. To maintain automatically full modulation of a carrier wave ofconstant amplitude for all variations of volume of the signals.

3. To maintain automatically full modulation of a carrier wave at anyamplitude of the carrier wave and for all variations of volume of thesignals.

4. To provide means which will require a minimum of power at thetransmitting station for the transmission of the desired signals.

The accompanying drawing shows a diagram of the transmission circuitsand contains some features which are similar to those shown in Fig. 1 ofmy co-pending application Serial Number 78,808, filed May 9, 1936. Inthis method or transmission the circuit of the microphone contains twotransformers, one of these is connected to a circuit which generateswaves having a frequency below the audible frequency and proportional tothe volume of sound received. These waves are fed back into themicrophone and they, together with the audible sound waves, aretransmitted through the other transformer to be amplified and used formodulation of a carrier wave. A particular feature of this invention isthe control of the amplification of the modulating current. This isshown in three stages of amplification, and means are shown forregulating this amplification according to a fixed voltage. which isrelated to the amplitude of the carrier wave. A receiving circuit forthis method of transmission may be similar to that shown in Fig. 2 ofthe above mentioned application. The drawing shows means for generatingcontrol circuits in the transmitting station which represent by theirfrequency the volume of sound received in the microphone. The

" currents produced in the microphone circuit, after being amplified,are rectified in the diode I0 and used to vary the frequency of theoscillator 19. These frequencies are combined with the frequency of anoscillator and the beats thus produced are used to generate sounds belowthe range of audibility in the loud speaker I08, which are fed back intothe microphone. The modulating current from the microphone circuit isconnected to the transformer I20 and is amplified in the circuits of thetubes I22, I31 and I52. This amplification is controlled to producecurrents of a substantially constant amplitude which are proportional tothe amplitude of the carrier.

In present methods of radio transmission the carrier wave is onlypartially modulated during most of the time it is being used fortransmission. This partial modulation is necessary in transmittingordinary signals in order to provide means for transmitting signals oflarger amplitude. This partial modulation represents a waste of powerwhich it is desired to save. In present practice it is customary toreduce the volume of the loudest signals, so that a carrier wave ofsufficient power to transmit these signals at their original volume willnot be required. It is also customary to amplify the weak signals sothat their amplification is greater than tne amplification of thosesignals which are stronger. But notwithstanding these contractions ofthe range of volume at the transmitting station, according to methodsnow in use, a large portion of the carrier wave is only partiallymodulated. The result is that much of the power used in generating thecarrier wave is notutiliaed when weak signals are being transmitted. Itis the purpose of this invention to save this power and still maintain aconstant strength of the carrier wave, unless it is desirable to changethe strength of the carrier wave on account of changing conditions whichwould affect transmission. Also a great deal of power is now used inamplifying the stronger signals. It is my purpose in this invention tosave power also in amplifying the stronger signals and to spend no morepower on the amplification to the strongest signals than is spent inamplifying those signals which are weakest.

To accomplish these purposes I have shown means by which all signalsfrom the strongest to the weakest are amplified to one strength which isonly strong enough to transmit the weak signals without interferencefrom noises, and atmospheric conditions. The amplitude of the carrierwave may be limited to an amount necessary to carry this amplitude ofmodulation. At the receiving set these signals are restored to theiroriginal variations in amplitude inthe method as shown in my formerapplication. Should it become desirable to use a stronger or a weakercarrier wave due to changes in daylight or changes in the atmosphere,the strength of the carrier wave may be changed correspondingly and theamount of modulation will be automatically changed to correspond withthe change in the strength to the carrier wave so that the carrier wavewill be fully modulated at whatever strength it is transmitted. Theprinciples shown in this invention for the control of the volume ofsignals are not limited to the transmission by signals by radio, but maybe used with other means of transmission.

In the accompanying drawing, the microphone circuit contains themicrophone I which receives the sound to be transmitted. a battery 2,and the primary coils of the transformers 3 and I20. The transformer I20is connected to circuits which modulate the carrier wave, and the.transformer 3 is connected to circuits which develop currents thefrequencies of which are proportional to the volume of the soundreceived in microphone I. The current from the secondary of transformer3 passes through the high pass filter I00 which filters out anyfrequencies below the audible range. The current from this filter I 00is amplified in the circuit containing the triode I02, batteries IOI and,I03 and transformer I04. It is then rectified in the circuit containingdiode I0, resistance 8 and condenser 9. The rectification of thiscurrent of audible frequency produces an envelope in which the amplituderepresents the variations in signal strength. The ends of thisresistance 8 are connected to the grid and filament of the triode I2.The circuit of this triode I2 amplifies the voltage variations in theenvelope produced by the diode I0, and this circuit contains the batteryII, the battery I4, the choke coil I3, and the saturation reactor I5.This saturation reactor is connected in the circuit of the oscillatingtube I9, which consists of the feedback coil II, the condenser I8,-thebattery 22 and the resistance 2I which is by-passed by the condenser 20.This oscillating circuit is adjusted to produce oscillations of mediumfrequency, the frequency of these oscillations are varied by thesaturation reactor I5, and are transmitted to the grid of the triode 33by connections to the transformer 23.

Another oscillating circuit consists of the triode 30, the feed backcoil 25, the condenser 27, the battery 3I and the resistance 29, whichis bypassed by the condenser 23. The circuit is adiusted to produceoscillations of about the same frequency as those produced by the tubeI9 and is connected to the grid of the triode 33 by the transformer 24.This circuit maintains a constant frequency.

The frequency of the beats between these two oscillating circuits variesas the frequency of the oscillations, produced bytube I9, is varied bythe saturation reactor I5. By proper adjustment of the two oscillatingcircuits the frequency of these beats may be made to vary inversely with.the amplitude of the direct current envelope and they may be confinedto a narrow band below the audible frequency. The oscillations of thesetwo-circuits are rectified by triode 33 in the circuit consisting of thebattery 32, the battery 46 and the condenser 34, and the beats aretransmitted through the transformer 35 and the low pass filter 36, whichfilters out any harmonics. These low frequency beats are connected tothe loud speaker I08, and thereby produce sounds below the audible rangewhich are received by the microphone I. The microphone circuit thencontains in addition to the signals which are to be transmitted lowfrequency signals which vary in frequency in proportion to the averagevolume of the signals being transmitted. These low frequency signals inthe microphone circuit which are produced by the sub-audible beats, arevolume control signals. They represent by their frequency the volume ofthe sound transmitted. In

quency sound waves produce low frequency signals in the microphonecircuit, which represent by their, frequency the volume of sound beingtransmitted. These low frequency signals in the microphone circuit,together with the sound signals in that circuit, are used to modulatethe I carrier.

As shown below, before these signals are used for modulation, theamplitude of the sound signals is regulated by reducing theamplification of the loud signals, and increasing the amplification ofthe weak signals until the amplitude of all of the sound signals issubstantially the same. Thus the carrier is modulated with two kinds ofsignals by the same modulating current. One kind of signal is ofsubstantially constant amplitude and represents by its frequency thefrequency of the sound being transmitted, and the other signals are ofsubstantially constant amplitude and represent by their frequency thevolume of ,the sound being transmitted.

These signals are transmitted to the transformer I20 through theconnections 43 and 44 and thence through the connections I and I58 tothe transmitter modulator, which is not shown.

The purpose of the intervening circuits is to regulate the volume of thesignals for the purposes mentioned above. amplitude to a constant valueand to fully modulate the carrier at all times, it is desirable toamplify these signals by comparison with a constant voltage,.or avoltage which is in constantratio with the amplitude of the carrierwave, and this is accomplished as follows.

The transformer 1 I20 is connected to the grid and filament of thetriode I22. This circuit of triode I22 contains the battery I23,condenser MI and transformers I24 and I25. The current from thetransformer I25 is rectified in the circuit containing the diode I20,resistance I26 and condenser I21.

The carriercurrent is generated by the alter- In order to hold this,

nator I33 and across the connections I51 and I58 of this circuit are theresistances I32 and I49. The wires I51 and I58 lead to the transmittermodulator. Connected to the ends of the resistance I32 is the circuit ofthe diode I3.I containing the resistance I29 and condenser I30. Thepositive ends of resistances I26 and I 29 are connected together. Thenegative end of resistance I29 is connected to the filament of thetriode I22 and the negative end of resistance I29 is connected to itsgrid. If the drop in voltage across the resistance I32 is greater thanthe voltage produced in the secondary of transamplitude of the signalcurrent may be amplifled so as to be more nearly constant. If thestrength of the carrier current is changed, the standard of comparisonfor the strength of the signal current is changed correspondingly.

This process may be repeated by the circuits of the triode I31 and thediodes I43 and I40. The circuit of the diode I43 is connected across theresistance I49 with the adjustable connection I30 and this circuitcontains the resistance I40 and the condenser I41. The circuit of thediode I40 contains the resistance I42 and the condenser HI and isconnected to the transformer I39 in the circuit of the triode I31.

The negative end of resistance I40 is connected by the wire I44 to thefilament of triode I31 and the negative end of resistance I42 isconnected by the wire I45 to the grid of triode I31. These lastmentioned circuits are used to correct any variations of amplitude whichmay not be made uniform by their amplification in the circuit of triodeI22, and the adjustment of connection I50 to the resistance I 49 is usedto adjust the amplification to obtain full modulation. The control ofthe amplification in the circuits of triodes I22 and I31 may be obtainedin a variety of ways. The resistances I29 and I46 may be connected toany other source of con stant voltage and be independent of theconnection to the current which produces the car rier wave. Thecondensers I21 and HI may-be made of such capacities as to regulate thespeed with which the change in rate of amplification follows a change ininput voltage in'the triodes I22 and I31. The condenser I21 and resistorI 26 may be set so that the circuit of diode I28 will respond withgreatest speed. With the re suit that if a peak of amplitude isimpressed on the grid of triode I22 the connection to the negative endof resistance I26 will make the grid less positive and reduce theamplification. The constant voltage supplied to the resistance I29 maybe adjusted so that the changes in amplification take out the peaks ofamplitude of the signal currents.

With the peaks of the current removed and the weaker signals amplifiedmore in proportion than the average signals, the current is impressed onthe triode I31. The condenser I4I maybe of such capacity that thevoltage applied to the grid and filament of triode I31 will respond moreslowly to the changes in the input voltage. With the result that thechanges in the rate of amplification in the triode I31 will followinversely the average changes in amplitude. Another method of adjustingthese circuits for controlling amplification is to fix resistors I26 andI42 and the capacities of the condensers I21 and I for the greatestfrequency of response in changing the rate of amplification. By thismeans the syllable variations in amplitude may be made uniform andspeech converts into an unintelligible mumble. The original amplitude Imay be restored by setting the control of am plitude in the receiver fora corresponding frequency of response. Such a receiver is shown in myapplication Serial No. 78,808 mentioned above. There will still be someslight variations in amplitude producing the voltagesin the cl:- cult ofI44 and I45 and these may be corrected by connecting these wires to thefilament and grid of triode I54. In this circuit consisting of triodeI52, battery I53, condenser II and transformer I54 the signals arefurther amplified and pass from thence through wires I55 and IE8 tophone are amplified and rectified and the transmitter modulator asmentioned above.

This method of transmission may be summarized as follows. The signalsfrom the microthrough a saturation reactor to an oscillating circuit ofmedium frequency as mentioned in my co-pending application. Thevariations in frequency produced in this oscillating circuit andcombined with another oscillating circuit producing beats of very lowfrequency which are transmitted to the microphone through the loudspeaker I00. The microphone circuit then contains the original signalsand low frequency signais below the audible range which vary infrequency according to the variation in volume of the signals to betransmitted.

The low frequency signals are eliminated from the triode I02 by the highpass filter I00, but are included in the connections to the transmittermodulator. The circuit to the transmitter modulator is amplified bycomparison with a constant voltage which is obtained across theconnection of the alternator producing the carrier wave. By amplifyingthe signals in comparison with a constant voltage and varying the amountof amplification so as to make the voltage corresponding to the strengthof the signals equal to the constant voltage, a constant amplitude ofsignals is obtained.

By reducing the signals to a constant amplitude, the carrier may alwaysbe fully modulated and the power for modulation and the power forgenerating the carrier wave may be limited to the power necessary totransmit signals free from interference by noises or atmosphericconditions.

At the receiving station the signals are restored to their originalvariations in amplitude by comparison with the variations in frequencyof the volume signals.

This invention is shown and described as referring to the transmisisonof electrical signals which represent sound signals. It is apparent thatit may be applied to electrical signals which represent other forms ofintelligence signals besides sound, and vit is intended to include thecontrol of electrical signals which are used to transmit intelligencesignals in any form.

I claim:

1. In a system for transmitting signals, an electrlcal transmittercircuit, means for generating waves of electrical energy in said circuitcorresponding to the signals being transmitted, means for generatingcontrol currents which vary in frequency with the volume of said signal,means for r'oducing inaudible sound waves corresponding in frequency tosaid control currents and means for generating waves of electricalenergy in said transmitter circuit corresponding to said inaudihis soundwaves.

2. In a system for transmitting electrical signals, means for carryingsaid signals, means for obtaining a predetermined reference voltage,means for rectifying said signals and producing a direct current voltagefrom said signals, the envelope of which varies in voltage with theamplitude of said signals, means for amplifying said signals, and meansfor increasing and decreasing the rate of said amplification bycomparing said variable voltage with said reference voltage.

3.v In a system for transmitting electrical sig- 'nalsiby radio, meansfor generating a carrier wave which is modulated by said signals, meansfor rectifying said signals and producing a direct said signals,meansfor amplifying said signals, means for rectifying said carrier waveand producing adirect current envelope which corresponds in voltage withthe amplitude of said car.- rier wave, means for using these twovoltages to regulate the amplification of said signals.

4. In a system for transmitting electrical signals; a transmitting meansof minimum capacity,

a means for obtaining a reference voltage which is proportional to thecapacity of said transmitnals, a transmitting means of minimum capacity,

a means for obtaining a reference voltage which is proportional to thecapacity of said transmitting means, means for producing a directcurrent voltage from said signals the envelope of which varies involtage with the amplitude of said signals, means for amplifying saidsignals, and means for reducing the amount of said amplification if saidvariable voltage is greater than said reference voltage.

6. In a system for transmitting electrical signals by radio, means forgenerating a carrier wave, means for obtaining a voltage which ispreportional to the amplitude of said carrier'wave, means forautomatically varying the amplitude of said signals so that all of saidsignals have substantially the same amplitude and means forautomatically regulating the amplitude of said signals so that saidamplitude has a fixed relation to said voltage.

7. The steps in a method of communicating signal energy of variableintensity which comprise generating a flow of electrical energy whichvaries in amplitude and frequency corresponding to the varyingcharacteristics of said signals in combination with a flow of electricalenergy which varies in frequency corresponding to the 1 variation involume of said signals, generating a carrier wave, developing a voltagewhich is proportional to the intensity of said carrier wave, amplifyingthis combined flow of electrical energy to a substantially constantamplitude which has a substantially fixed relation to said voltage andproducing a modulation of substantially constant amplitude in saidcarrier wave with said combined flow of electrical energy.

8. The steps in a method of communicating signal energy of variableintensity which comprise generating a flow of energy which varies inamplitude and frequency corresponding to the varying characteristics ofsaid signals in combination with a flow of electrical energy whichvaries in frequency corresponding to the variation in volume of saidsignals, generating a carrier wave, developing a voltage which isproportional to the intensity of said carrier wave, automaticallyamplifying said combined flow of electrical energy with a varying degreeof amplification which is inversely proportional to the varyingintensity of said fiow of energy until the intensity of said combinedflow of electrical energy is more nearly constant and which issubstantially equal to said proportional intensity and modulating saidcarrier wave with said combined flow of energy of modified intensity.

9. The steps in a method of communicating signal energy of variableintensity which com-v prise generating a now. of which varlesinamplitude and frequency cojto the varying characteristics of said claimsin combination with a fiow of energy which varies in frequencycorresponding tothe variation in volume of said signals generatinga'zcarrlerwave'.

developing an intensity which is proportional to the intensity of saidcarrier wave, automatically amplifying said combined fiow of energy witha varying degree of amplification which produces in said flow of energya transformation of its constant intensity whichis proportional to theintensityof a carrier medium, amplifying the intensity of said signalenergy, automatically raising said rate of amplification to bring theintensity of said signals up to said constant in-.

tensity if the intensity of said signal energy is less than saidconstant intensity and decreasing said amplification if the intensity ofsaid signal energy is greater than said constant intensity.

and modulating said carrier medium with said' signal energy of modifiedintensity. I I

11.'The steps in a method of communicating signal energy of variableintensity, which comprise, generating 'a substantially constant carrierwave, and obtaining a voltage from said car.-

rier wave which is proportional in intensity to the intensity of saidcarrier wave, amplifying said signal energy of variable intensity, andautomatically increasing the amplification of said variable intensitywhen it is less 'than said voltage which is proportional to said carrierwave.

12. In a s'ystem'for transmitting electrical sig-' nals, atransmittingmeans; for said signals of minimumwapacity, means for amplifying saidsignals. means for rectifying said signals, means for producing avoltage which represents thevariations in amplitude of said signals,means for obtaining a source or substantially constant voltage which hasa definite relation to the capacity of said transmitting means forcomparing the voltage produced by said signals with said constantvoltage and means for' contracting the variation in amplitude of's'a'idsignals by raising the rate of said amplification to bring saidrepresentative voltage up to said constant 'voltageand decreasing saidrate of amplification, according as to whether the voltage representingsaid signals is less or greatcrthan said constant voltage.

a 13. In a system for transmitting electrical sig--v nals, atransmitting means of minimum capacity, means for producing'a directcurrent voltage from said signals, the envelope of which varies meansfor obtaining a substantially constant voltage which has a definiterelation to the ca-.

pacity of said transmitting means, means for amplifylng said signals andmeans for automatically raising the rate of said amplification toincrease the voltage representing weak signals if said variable voltageproduced by said weak signals is less than said constant voltage.-

14. A method of transmitting intelligence signals of variable intensity,which comprises, gen- 'in voltage with the amplitude of said signals,

erating a carrier wave of substantially constant frequency, obtaining anintensity of voltage proportional to the intensity of said carrier,generating control signals which represent by their frequency theintensity of said intelligence signals, amplifying said intelligencesignals, varying the rate of said amplification so that all signals ofvariable intensity are amplified to substantially equal saidproportional intensity which is proportional to the intensity of saidcarrier wave, modulating said carrier wave with currents which containsaid intelligence signals of substantially constant intensity and saidcontrol signals.

15. A method of transmitting signals of variable frequency and variableintensity, which comprises, generating a carrier wave, obtaining avoltage proportional to the amplitude of said carrier, generating wavesof electrical energy flow containing frequencies which vary according tothe variable frequencies of said signals, and containing frequencieswhich represent by their variation the variable intensity of saidsignals,- amplifying said flow of electrical energy, varying the rate ofsaid amplification to produce an amplitude which has a fixed relation tosaid proportional voltage in said flow of electrical energy, modulatingsaid carrier wave according to the characteristics of said energy flow.

16. In a system for transmitting electrical signals by radio, means forgenerating a carrier wave, means for obtaining a voltage whichrepresents a definite proportion of the amplitude of said carrier wave,means for amplifying said signals, mears for rectifying said signals andobtaining a voltage which represents the amplitude of said signals,means for comparing said voltages, means for automatically regulatingthe amplication of said signals to approximately equal said proportionof the amplitude of said carrier wave by the difierence between saidvoltages.

17. The steps in a method of communicating signal energy of variableintensity, which comprise, generating a flow of electrical energy whichcontains variations in amplitude corresponding to the varyingcharacteristics of said signals, and which contains variations infrequency correa spending to the variations in the volume of saidsignals, generating a carrier frequency wave, oh-

taining a voltage proportional to the amplitude of said carrier wave,amplifying said fiow of electrical energy to a definite relation to saidproportional voltage, and modulating said carrier wave with saidcombined flow of electrical energy.

18. In a system for transmitting electrical signals by radio, means forgenerating a carrier frequency wave, means for generating a constantvoltage, which is proportional to the amplitude of said carrier wave,means for amplifying said signals, means for obtaining a voltage whichis proportional to the amplitude of said signals, means for obtainingthe differential of said voltages, means for varying the amplificationof said signals so that all of said signals have substantially the sameamplitude, and means for regulating said amplitude of said signals bythe differential of said voltages.

19. A method of transmitting signals of variable frequency and variableintensity, which comprises, generating a carrier wave, obtaining anintensity of voltage proportional to the intensity of said carrier wave,generating alternating currents which have an intensity substantiallyequal to said proportional intensity, and which represent by theirfrequency said variable frequency of said signals, generatingalternating currents which represent by their frequency the variableintensity of said signals, modulating said carrier wave with saidalternating currents.

20. A method of transmitting signals of variable frequency and variableintensity, which comprises, generating a carrier wave, obtaining anintensity of voltage proportional to the intensity of said carrier wave,producing waves of electrical energy flow substantially equal to saidproportional intensity which are equivalent in frequency to the variablefrequency of said signals, producing waves of electrical energy flow ofsubstantially constant intensity which have a variation in frequencyrepresentative of the variations in intensity of said signals,modulating said carrier wave in accordance with the variations infrequencies of said flows of electrical energy.

JAMES L. GE'IAZ.

